In recent years, roller bearings such as fluid bearings and oil-impregnated sintered bearings have often been employed as bearings of spindle motors for a magnetic disk, an optical disk, etc., since such roller bearings realize silent operation and durability.
In the above bearings, a shaft is not in direct contact with the bearing inner surface but is in contact therewith by the mediation of lubricant, which sustains a load applied to the shaft, thereby reducing friction between the shaft and the bearing.
Thus, the performance of the bearings greatly depends on the performance of lubricating oil.
Requirements in performance of the lubricating oil for use in the roller bearings include viscosity, durability, and antistatic property.
Among these requirements, viscosity is an essential factor for determining electric power loss and bearing toughness of a spindle motor. In a recent year-by-year trend toward increasing rotation speed (to a range of 10,000 to 50,000 rpm) of spindle motors for use in information-related apparatuses (particularly CD apparatuses, DVD apparatuses, HDDs, and laser printers (polygon mirror), a lubricating oil of low viscosity is selected so as to reduce electric power loss during high-speed operation.
In general, when the viscosity of lubricating oil decreases, the amount of vaporized oil increases.
Thus, when a low-viscosity lubricating oil is employed, loss of the lubricating oil increases, resulting in a lubrication failure, and in the worst case, damage to bearings.
Under such circumstances, there have been proposed a large number of lubricating base oils for bearings satisfying both low viscosity and low volatility; for example, ester compounds (Japanese Patent Application Laid-Open (kokai) No. 11-315292, p. 1), monoesters (ditto No. 2000-63860, p. 1), carbonate esters (ditto No. 2001-107046, p. 1), use of poly(α-olefin) and an ester in combination (ditto Nos. 2001-172656, p. 1 and 2001-240885, p. 1), use of a diester and a polyol ester in combination (ditto No. 2001-279284, p. 1), neopentyl glycol esters (ditto No. 2001-316687, p. 1), an aromatic ester or diester (ditto No. 2002-97482, p. 1), monoesters (ditto No. 2002-146381, paragraph [0007]), and specific diesters formed from oxalic acid, malonic acid, succinic acid, etc. (ditto No. 2002-155944, p. 1).
Meanwhile, in a roller bearing, a shaft and a bearing are completely separated by lubricating oil film.
Therefore, static electricity tends to be generated by movement of fluid. When the thus-generated electricity is discharged, essential electronic parts and magnetic parts (e.g., MR head of a hard disk drive) may become disordered.
Thus, static charge of roller bearings for use in precision machines such as a magnetic disk apparatus must be caused to flow to ground, to thereby protect electronic and magnetic apparatuses against static electricity.
From this viewpoint, the aforementioned conventional bearing lubricating oils still have the problem that they have problematically large volume resistivity, which readily induces generation of static electricity, although they satisfy both low viscosity and low volatility.
In order to solve the above problem, a lubricating oil to which conductive microparticles of a metal or a metal oxide have been added is reported (see, for example, Japanese Patent Application Laid-Open (kokai) No. 10-30096, p. 1 or 11-315292, paragraph [0023]). However, when a lubricating oil containing such microparticles is employed, microparticles present on the sliding surface cause anomalous wear of the bearing during start and stop of the motor.
A lubricating oil containing a metal organic salt such as sulfonate, phenate, or salicylate, instead of such metallic particles, has also been proposed (see Japanese Patent Application Laid-Open (kokai) 2001-234187, p. 1).
However, the above metal organic salt antistatic agent can exhibit antistaticity only when added in a large amount.
In addition, during long-term use of the lubricating oil, the antistatic agent problematically forms an inorganic salt (sludge), which is insoluble in oil.
The present invention has been accomplished in an attempt for solving the aforementioned problems, and an object of the present invention is to provide a conductive lubricant composition which, without impairing roller bearing oil performance, prevents anomalous wearing, generation of sludge, and occurrence of static electricity which would otherwise be generated by movement of fluid.